1. Field of the Invention
The present invention relates to a semiconductor device. In particular, the present invention relates to a clamp diode used in a semiconductor device.
2. Description of the Related Art
A clamp diode is a device which uses a junction breakdown voltage between the p-type semiconductor and the n-type semiconductor to maintain (clamp) a voltage supplied to a circuit constant in a semiconductor device. Since a clamp diode is a device capable of limiting a voltage easily, there is a high demand and a wide usage in electronic devices.
When manufacturing a clamp diode, which limits a voltage to be constant, it is extremely important for the clamp diode to have small fluctuations in breakdown voltage within a wafer, among wafers, and among lots, and to have little change over time. In addition, it is also important for the clamp diode to have a small leakage current until breakdown occurs. Even though the structure of the clamp diode is simple, it is not easy to produce a clamp diode which satisfies all of the above-mentioned characteristics.
Japanese Published Patent Application No. 11-307787 discloses the invention for improving the above-mentioned change over time. FIG. 7 is a cross-sectional view of the structure illustrated in FIG. 1 of the Japanese Published Patent Application. There is described that, as illustrated in FIG. 7, a second conductivity type high concentration region 1 is placed away from an element isolation insulating film 2 by a predetermined distance, and further, an electrode 8 is provided through the intermediation of an insulating film 9, thereby adjusting a voltage of the electrode 8 to improve the change in the clamp diode over time. FIG. 8 is a cross-sectional view of the structure illustrated in FIG. 6 of the Japanese Published Patent Application. As shown in this structure, there is described that the same effect can be obtained even without the electrode 8 of FIG. 7.
Though the change over time can be improved by the invention disclosed in the Japanese Published Patent Application, there is no description, however, on the degree of fluctuations within a wafer, between wafers, and among lots, and the presence or absence of leakage before breakdown occurs. Indeed, in the invention disclosed in the Japanese Published Patent Application the fluctuations within a wafer, between wafers, and among lots are not reduced because of the following reasons.
Since an electric field corresponding to a voltage applied to the electrode 8 is applied to a p-n junction through the oxide film 9 in the structure illustrated in FIG. 7, the electric field is thought to be constant and the p-n junction breakdown voltage does not seem to fluctuate due to the electric field. Ideally, when the same voltage is applied to the electrodes 8, the breakdown voltages are the same. However, actually, the thicknesses of the oxide films 9 are not the same within a wafer, between wafers, and among lots, and hence the electric fields applied to the p-n junctions by the voltages of the electrodes 8 fluctuate. As a result, the p-n junction breakdown voltages fluctuate.
Further, due to the presence of a first conductivity type region 7 below the element isolation insulating film 2 of FIGS. 7 and 8, impurities in the first conductivity type region 7 affect impurity distribution near the p-n junction, and the breakdown voltage of the p-n junction changes. Also in this regard, ideally, when the first conductivity type regions 7 are produced in the same condition, the breakdown voltages of the p-n junctions are the same. Actually the concentrations of impurities in the first conductivity type regions 7, however, are not the same within a wafer, between wafers, and among lots, and the p-n junction breakdown voltages are affected to various degrees. As a result, the p-n junction breakdown voltages fluctuate.
According to the Japanese Published Patent Application No. 11-307787, a planar shape of the second conductivity type high concentration region 1 of FIG. 7 is octagon (not shown in this specification). In such a structure having corners, electric field strength is high at a corner portion, and therefore the breakdown voltage is determined by the corner portion. FIG. 9 shows the breakdown voltages in the case where the planar shape of the second conductivity type high concentration region 1 is rectangular and in the case where the planar shape thereof is circular. It is apparent that the breakdown voltage is lower in the case of the clamp diode having the rectangular second conductivity type high concentration region 1 than in the case of the clamp diode having the circular second conductivity type high concentration region 1. That is, it is apparent that the electric field is concentrated at the corner portions of the rectangle, and the corner portions determine the breakdown voltage. Ideally, the breakdown voltages of the p-n junctions will be the same since the shapes of the corner portions are always the same when lithography is performed in the same condition. Actually the electric field strength at the corner portions, however, fluctuates within a wafer, between wafers, and among lots. As a result, the p-n junction breakdown voltages fluctuate.
As described above, there are many factors that cause the breakdown voltages of the p-n junctions to fluctuate, and, in order to suppress such fluctuations, the clamp diode needs to have a structure as simple as possible, and the factors that cause fluctuations need to be eliminated as much as possible.